It is well known that gas to be consumed in cylinders of an internal combustion engine, is drawn therein through an inlet manifold, past a speed control throttle. The gas usually comprises a fuel/air mixture produced by a carburettor but the gas can comprise air alone if the engine includes a fuel injection system.
In operation of the engine, the throttle causes a partial vacuum to be established in the inlet manifold, the level of partial vacuum being a function of the load of the engine. More particularly, when the engine is idling under substantially no load, the throttle is almost closed and causes a reduction in the pressure of gas in the inlet manifold, but when the throttle is opened to permit the engine to work against an increased load, the gas pressure in the inlet manifold increases thereby decreasing the level of partial vacuum.
It is known to provide a vacuum transducer arranged to sense the level of partial vacuum in the inlet manifold, to provide an indication of engine load, and such a transducer has been used with engines having a spark ignition system to control the timing of the spark ignition system as a function of engine load.
When the engine is in dynamic equilibrium, the partial vacuum in the inlet manifold constitutes an accurate indication of engine load. However a major disadvantage of the known vacuum transducer arrangement is that during operation of the speed control throttle to change the engine load, the engine moves out of dynamic equilibrium which results in the inlet manifold pressure lagging behind the value it would assume under steady state engine conditions to reflect accurately the engine load. Consequently, the output of the vacuum transducer becomes delayed and inaccurate during changes in engine load.